Nitride semiconductors, which have a direct-transition-type wide band gap, high breakdown electric field, and high saturation electron velocity, have been used as light emission devices such as LED or LD and semiconductor materials for high-frequency/high-power electronic devices.
Typical structures of the nitride electronic devices include a high electron mobility transistor (HEMT) structure which is formed by laminating AlGaN as “a barrier layer” and GaN as “a channel layer”. This structure utilizes a feature that a high concentration two-dimensional electron gas is generated at an AlGaN/GaN lamination interface owing to large polarization effects (spontaneous polarization effect and piezo polarization effect) inherent in nitride materials.
The nitride electronic devices are generally manufactured using different material base substrates such as sapphire, SiC, and Si which are easily available in a commercial way. However, there arises a problem that large numbers of defects occur in a GaN film which is heteroepitaxially grown on the different material substrates due to a difference of lattice constant and heat expansion coefficient between GaN and the different material substrates.
In the meanwhile, when the GaN film is homoepitaxially grown on a GaN substrate, the defect caused by the difference of lattice constant and heat expansion coefficient described above does not occur, but the GaN film shows a favorable crystalline nature.
Accordingly, when the nitride HEMT structure is manufactured on the GaN substrate, the mobility of the two-dimensional electron gas at the AlGaN/GaN lamination interface is enhanced, thus a characteristic improvement of an HEMT element (semiconductor element) manufactured using the above structure can be expected.
However, the GaN substrate manufactured by a hydride vapor phase epitaxial growth method (HVPE method), which can be commercially available, generally has an n-type conductivity due to an oxygen impurity incorporated into a crystal. The conductive GaN substrate serves as a leakage current pathway between source-drain electrodes when the HEMT element is driven at high voltages. Thus, it is preferable to use the semi-insulating GaN substrate to manufacture the HEMT element.
It is known to be efficient to perform doping of an element such as a transition metal element (Fe, for example) or group 2 element (Mg, for example) which forms a deep acceptor level in the GaN crystal to achieve the semi-insulating GaN substrate.
It is already known that when zinc (Zn) is adopted from a group 2 element, a high-quality semi-insulating GaN single-crystal substrate can be achieved (for example, refer to Patent Document 1). An investigation has been already performed on diffusion of the Zn element in the GaN crystal, and the diffusion occurs in a high temperature atmosphere and the ease of diffusion depends on the crystallinity of the GaN crystal (for example, refer to Non-Patent Document 4.) Also known is an aspect that a high-resistance layer doped with iron (Fe), which is a transition metal element, is formed on a substrate, and an intermediate layer having a high effect of incorporating Fe is further formed between the high-resistance layer and an electron transit layer, thereby preventing Fe being incorporated into the electron transit layer (for example, refer to Patent Document 2).
The manufacture of the HEMT structure on the semi-insulating GaN substrate or a substrate with the semi-insulating GaN film to evaluate each characteristic has been already performed (for example, refer to Non-Patent Document 1 to Non-Patent Document 3).
When the GaN film is epitaxially grown on the semi-insulating GaN single crystal substrate, which is doped with the transition metal element or the group 2 element, to form an epitaxial substrate for the semiconductor elements, there arises a problem that an acceptor element such as Fe, Mg, and Zn is diffused in the GaN film and acts as an electron trap in the film, thus a current collapse phenomenon occurs (for example, refer to Patent Document 3). Patent Document 3 discloses that the acceptor element, such as Fe and Mg, which is easily diffused causes current collapse.